1. Field of the Invention
This invention relates to a modified alternan having a lower apparent molecular weight than native alternan and to a method for producing the modified alternan. The modified alternan is produced by cultivation of native alternan in the presence of a Penicillium sp. isolate capable of essentially quantitative conversion of the native alternan to a polymeric modified form. The rheological properties of the modified alternan are similar to commercial gum Arabic.
2. Description of the Prior Art
The polysaccharide alternan was first described by Jeanes et al. (1954, J. Am. Chem. Soc., 76:5041-5052) as one of two extracellular α-D glucans, referred to as fraction S, produced by Leuconostoc mesenteries NRRL B-1355. The structure of this fraction was later determined by Misaki et al. (1980, Carbohydr. Res., 84:273-285) to consist primarily of an alternating sequence of α-1,3-linked and α-1,6-linked D-glucose residues, with approximately 10% branching. Because the α-1,3-linkages are part of the linear chain of the S fraction and there are not any consecutive α-1,6- linkages, the fraction is not a true dextrin, and Cote and Robyt (1982, Carbohydr. Res., 101:57-74) therefore named this fraction alternan. These authors also isolated the enzyme alternansucrase which synthesizes alternan from sucrose.
Native alternan has an apparent molecular weight average ( M w) of 106-107 [Cote G. L., (1992), Carbohydrate Polymers 19:249-252; Cote G. L. et al., (1997), Alternan and highly branched limit dextrans: Low-viscosity polysaccharides as potential new food ingredients. In: Spanier A. M. et al. (ed) Chemistry of Novel Foods, Carol Stream, IL.: Allured Publishing Corp, pgs. 95-110]. Derivatives of alternan have been produced by ultrasonication, reducing the apparent molecular weight average to <106 and modifying the rheological properties of the polymer so that they more closely resemble gum Arabic [Cote, 1992, Ibid]. However, ultrasonication is a relatively expensive process that would be difficult to carry out on an industrial scale. Although alternan is resistant to hydrolysis by most known endoglucanases, a “limit alternan” has also been produced by treatment of native alternan with isomaltodextranase from Arthrobacter globiformis [Cote, 1992, Ibid]. Limit alternan exhibited an apparent M w of 3.5×103 and was rheologically similar to oligosaccharides of maltodextrin [Cote, 1992, Ibid]. Recently, Bacillus sp. isolates were described that produced an endoglucanase specific for alternan [Biely P. et al., (1994), Eur. J. Biochem., 226:633-639; Cote G. L. et al., (1998), U.S. Pat. No. 5,889,179; Wyckoff H. A. et al., (1996), Curr. Microbiol., 32:343-348]. This enzyme produced a novel cyclic tetrasaccharide from alternan [Cote G. L. et al., (1994), Eur. J. Biochem., 226:641-648].
The desire to further extend the range of properties of modified alternan and to expand the scope of new applications for these materials has led to a continued search for new organisms, particularly fungi, that could partially degrade or modify the native polysaccharide.